1. Field of the Invention
The present invention relates to a starting circuit of a single-phase induction motor.
2. Description of the Related Art
A known starting circuit of a single-phase induction motor used in a compressor of an electric refrigerator includes a motor starting positive characteristic thermistor and a triac connected in a serial connection which is in turn connected in series with an auxiliary coil for starting the motor. A triac control positive characteristic thermistor is connected in parallel with the motor starting positive characteristic thermistor. One terminal of the triac control positive characteristic thermistor is connected to a gate terminal of the triac.
In the starting circuit of the single-phase induction motor, a trigger signal, applied to the gate terminal of the triac through the triac control positive characteristic thermistor, causes the triac to be conductive. A motor starting current thus flows through the auxiliary coil via the motor starting positive characteristic thermistor. After a predetermined period of time subsequent to the startup of the motor, the motor starting positive characteristic thermistor reduces the current flowing through the auxiliary coil in response to an increase in resistance resulting from a temperature increase therein. The triac control positive characteristic thermistor reduces the current applied to the gate terminal of the triac in response to an increase in resistance resulting from a temperature increase therein, to thereby turn off the triac. Since the triac control positive characteristic thermistor has a smaller thermal capacity than the motor starting positive characteristic thermistor, power consumption is relatively small.
The starting circuit for the single-phase induction motor has difficulty in properly controlling the current so as to be cut off within a predetermined period of time (typically 1 to 10 seconds) within which the motor completes starting under varied operating temperature conditions. Under a low temperature environment, it takes a relatively long time for the triac control positive characteristic thermistor to reach a high resistance value by self-heating. On the other hand, under a high temperature environment, the resistance is already too high for the motor to operate normally.
Japanese Unexamined Patent Application Publication No. 9-285168 discloses a motor starting circuit. In the disclosed motor starting circuit, a resistance of a triac control positive characteristic thermistor at a temperature of 25° C. (hereinafter referred to as R25) falls within a range of 300 Ω to 3000 Ω, the volume of the triac control positive characteristic thermistor ranges from 30 mm3 to 60 mm3, and the temperature of the triac control positive characteristic thermistor which produces a resistance of twice R25 (the Curie temperature) ranges from 70° C. to 125° C. The disclosed circuit reliably cuts off a current flowing through an auxiliary coil in a predetermined period of time within which a motor completes starting, regardless of variations in operating temperature. Power consumption subsequent to the startup of the motor is reduced. Japanese Unexamined Patent Application Publication No. 6-339291 discloses a technique in which reducing the volume of a triac control positive characteristic thermistor reduces power consumption.
Currently, demands for lower power consumption and miniaturized component parts are increasing. To further reduce power consumption, miniaturization of the triac control positive characteristic thermistor is effective. In accordance with the above-disclosed techniques, reducing the volume of the triac control positive characteristic thermistor to below 30 mm3 is difficult. FIG. 2 shows a plot of the volume versus cutoff time relationship of a triac control positive characteristic thermistor having the Curie temperature of 70° C. at an operating temperature of 100° C. As shown in FIG. 2, the cutoff time greatly varies as the volume is reduced to less than 30 mm3. With such variations, reliable operation of the motor is difficult.